Use ring structure to access the cpu/gpu address of rptr/wptr.

v2: merge gfx10/sdma5/sdma5.2 patches
Signed-off-by: NJack Xiao <Jack.Xiao@amd.com>
Reviewed-by: NChristian König <christian.koenig@amd.com>
Reviewed-by: NHawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>